1. Field of the Invention
This invention relates to a pressure relief valve (check valve), particularly of the type in which an adjustable spring urges a movable valve member (hereafter "valve head") into the closed (shutoff) position, and the valve head is lifted when the fluid pressure acting on the valve head against the spring force overcomes the latter and lifts the valve head off its seat into an open position, permitting flow of fluid through the valve. By virtue of the adjustability of the valve-closing spring, the valve may be set to open at a predetermined pressure (set pressure), thus limiting the fluid pressure to a desired value in the conduit upstream of the pressure relief check valve.
Pressure relief valves of the above-outlined type may find advantageous application in fluid pump circuits in which the pump draws fluid from a reservoir and drives the fluid to a user circuit (output line). Typically, the output side of the pump is connected with the intake side (reservoir) by a bypass circuit which contains a normally open globe valve to bypass flow around the pump. Such a circuit has a significant safety function inasmuch as in its absence any blockage of the output line (for example, an upstream pump closure) could cause a pressure buildup resulting in a dead-heading of the pump with high risks of damage thereto. Such a dead-heading is prevented by the bypass circuit in that the globe valve is either open or set to open if its back pressure exceeds a predetermined value. It is known, however, that globe valves can be inadvertently closed during routine maintenance with very little likelihood of being detected before the system resumes operation. Also, it is known that where a remote sensing device is used to open the globe valve at a preset pressure, failure of the sensing device could render the globe valve inoperable. In either of these two cases, dead-heading will occur upon an upstream blockage of the output line.
2. Description of the Prior Art
Conventional piston-type and lift-type check valves are known where the valve closing force is supplied by a coil spring arrangement, assisted by fluid pressure. For this purpose, the spring urges the piston-like valve head against its seat and further, a fluid port maintains communication between a chamber (which usually accommodates the spring) and the upstream side of the valve head to ensure that fluid under pressure is admitted from the inlet side of the valve into the chamber behind the valve head. The surface areas on the two sides of the valve head, exposed to the fluid pressure are of such a magnitude and are so oriented that the force derived from the fluid pressure is smaller in the closing direction than in the opening direction of the valve head.
When the fluid pressure exceeds a certain predetermined limit value, the opening force derived from the fluid pressure will be greater than the combined force of the spring and that derived from the fluid pressure behind the valve head, causing the latter to lift off its seat, resulting in a fluid flow through the valve. Due to the fluid flow, the pressure conditions change: while the valve is lifted by the effect of static pressure, it is maintained open substantially by dynamic pressure and further, the fluid pressure behind the valve head, seeking to close the same, is altered by the appearance of the dynamic pressure. Conventional arrangements are not adapted to adequately isolate the pressure chamber behind the valve head from flow turbulences that appear as the fluid passes through the opened valve and are also not adapted to develop a desirable high static pressure behind the valve head. Rather, the pressure in the chamber behind the valve head will be effected by line velocity (and thus by significant dynamic pressure) and turbulences, preventing the development of a stable, highly static pressure.